1. Field of the Invention
The present invention relates to optical wavelength conversion elements, methods of manufacturing optical wavelength conversion elements, wavelength conversion devices, ultraviolet laser irradiation apparatuses, and laser processing systems.
2. Discussion of the Background
In an ultraviolet laser, a method is employed in which ultraviolet light is generated from a source laser that emits light in the infrared region through multiple uses of a nonlinear optical effect of an optical wavelength conversion element. In the method, for an optical wavelength conversion element that is used in, for example, the final stage, a cesium-lithium-borate crystal that is typified by a crystal with a chemical composition, CsLiB6O10, (hereinafter also referred to as “CLBO”) and that was developed by the present inventors is a crystal capable of generating an ultraviolet laser highly efficiently. It has been reported that the CLBO crystal can generate, for example, fourth harmonic (with a wavelength of 266 nm) and fifth harmonic (with a wavelength of 213 nm) of a neodymium-doped yttrium aluminium garnet (Nd:Y3Al5O12 hereinafter also referred to as Nd:YAG) laser as well as light with a wavelength of 193 nm that is equal to the emission wavelength of an ArF excimer laser, with high conversion efficiency.
However, since the CLBO crystal is deliquescent, there is a problem in that the crystal optical surface reacts with, for example, moisture in the atmosphere, which results in a deterioration in quality or laser damage and also causes a change in internal refractive index. Accordingly, a technique has been developed in which the crystal is pretreated by being heated (annealed) at 100° C. or higher before it is used as an optical wavelength conversion element, or it is used in the state where it is heated at 150° C. (as described in JP 3115250 B, which is incorporated herein by reference in its entirety). Furthermore, there are the following proposals. That is, in order to prevent the optical wavelength conversion element from being deteriorated during the use, the crystal is disposed in a vacuum atmosphere (as described in JP 11(1999)-271820 A, which is incorporated herein by reference in its entirety), or the crystal used is disposed inside an optical cell sealed with a gas free from moisture (as described in JP 2003-295241 A or WO 2002/048786, each of which is incorporated herein by reference in its entirety). All of these techniques are intended to prevent element deterioration and laser damage that are caused by deliquescence of the CLBO crystal.
On the other hand, in an optical wavelength conversion element produced using a CLBO crystal, when the power of generated ultraviolet laser increases, a slight absorption thereof causes the inner part of the element to generate heat to form a nonuniform temperature distribution therein. This gives rise to distribution of changes in refractive index, which results in a region that departs from the wavelength conversion condition (phase matching condition). Therefore, in order to solve both the problems of the aforementioned moisture prevention and the generation of the phase mismatch region, there is a method in which a CLBO crystal is used in the state where it is heated at 150° C. However, since in this method, it is difficult to avoid the effect of self-heating, the output power obtained when a high output ultraviolet laser is generated tends to be lower than the theoretical value obtained with no consideration given to the heat effect. Accordingly, in order to increase the output power of an ultraviolet laser, input of a higher power laser and conditions for highly condensing light are indispensable. This, however, works as a factor that causes damage and deterioration of the CLBO crystal, and thereby reliability of long-term operation is deteriorated, which is a problem. The vicinity of the laser beam condensing part inside the crystal is subjected to an environment where an “internal laser damage” in which heat cracks occur due to the heat generated by the ultraviolet laser tends to be caused. Furthermore, since the CLBO crystal has the property that the refractive index value decreases with heat generation, the CLBO crystal exhibits a “thermal lens effect” in which the diameter of a laser beam light collected for improving the ultraviolet laser conversion efficiency disadvantageously increases. Moreover, the use of a CLBO crystal in the state where it is heated at 150° C. requires time for starting up (activating) the ultraviolet laser irradiation apparatus and therefore there has been a problem in operability. Thus there have been demands for the development of a CLBO crystal that can be used at room temperature.
On the other hand, it has been known that water, contained as an impurity in a CLBO crystal, causes absorption in an infrared light region (as described in Y. Morimoto et al., J. Mater. Res. Vol. 16, pp. 2082-2090 (2001) or L. Kovacs et al., Opt. Mater. Vol. 24, pp. 457-463 (2003), each of which is incorporated by reference herein in its entirety). In response, the present inventors developed a technique for reducing the water impurities inside the CLBO through a heat treatment to improve the threshold of the CLBO crystal internal laser damage (as described in M. Nishioka et al., Jpn. J. Appl. Phys. Vol. 44, pp. L699-L700 (2005), which is incorporated by reference herein in its entirety). However, the CLBO crystal produced by this technique has a problem in that an output saturation phenomenon occurs in which the output power of ultraviolet laser generated also increases up to a certain region with an increase in source laser input power, but when the input power exceeds a certain value, the output power stops increasing. Accordingly, there are demands for the development of a technique for improving the output power of ultraviolet laser according to the input power. Such use at room temperature and improvement in ultraviolet laser output also are problems to be solved in all the cesium-lithium-borate crystals including the CLBO crystal.